Recoloring of symbolic icons

[El Boulangero <elboulangero gmail com>]

Hi Gtk+,

Short story:

In my application, I have use custom symbolic icons, and I want to
recolor them with the css.

What I observe right now is that, as long as my icons are loaded from a
svg file, they are not recolored and remain grey. However, if I render
my icons to png with 'gtk-encode-symbolic-svg', these png files are
loaded instead, and recolored.

I'm wondering if this behavior is expected, as I couldn't find any
information on that matter.

----

Long story:

I'm building an application in which I have dice buttons. Depending if
the button is clicked or not, I want to change the color of the
background of the dices, and also the color of the dots on the dices.

I'm trying to implement that by using symbolic icons. I have svg dice
icons, and typically a dot looks like that in the svg file:

    <circle cx="117" cy="440" r="70"
style="display:inline;fill:#bebebe;fill-opacity:1;stroke:none"/>

My icon is named 'dice-1-symbolic.svg', I load it with a call like this one:

    Gtk.Image.new_from_icon_name('dice-1-symbolic', size)

I have a css that looks like that for my dicebutton.

    .dicebutton {
      color: #ffffff;
      background-color: #6199ff; }

The idea is to recolor the dices by changing the css attributes.

- What works: recoloring the background (aka changing 'background-color').
- What doesn't work: recoloring the icon (aka changing 'color' has no
effect).

I tried again and again, and what I observe at the moment is that:
- as long as I use a svg file as the source, the icon is not recolored
- if, however, I render my svg to png using 'gtk-encode-symbolic-svg',
the png is picked up instead of the svg, and the icon is then recolored
according to the CSS.

That's a bit puzzling to me. Moreover, my naive understanding was that a
SVG was trivial to recolor (since it's just about changing the colors
definitions in the file), while PNG was harder to recolor, since it's
binary data. So, I thought that SVG were recolored first, then rendered
to PNG with the right color.

But it looks like it doesn't work like that, especially since I
discovered that the icon theme on my system is full of '.symbolic.png'
files, which makes me wonder, what's the point with the svg files in the
end ? Are they used at all by Gtk ?

So, I'm wondering if anyone could enlighten me a bit about how gtk
handles this re-coloring. I didn't find the right piece of information yet.

In the end, should I always render my svg to png ? Or is there a way to
have my svg recolorized by the css ?

Any advice welcome !

Cheers,
Arnaud